Motion-corrected pet images

ABSTRACT

A method is disclosed for generating a motion-corrected PET image of an examination area in a combined MR-PET system. In an embodiment, the method includes recording PET events from the examination area in a first recording time frame; recording a number of MR images of the examination area in at least the first recording time frame; calculating an at least two-dimensional movement information of the examination area on the basis of the number of MR images, wherein the movement information describes the movement information of the examination area during the first recording time frame, and determining the motion-corrected PET image from the PET events using the calculated movement information.

PRIORITY STATEMENT

The present application hereby claims priority under 35 U.S.C. §119 toGerman patent application number DE 102012218289.6 filed Oct. 8, 2012,the entire contents of which are hereby incorporated herein byreference.

FIELD

At least one embodiment of the invention generally relates to a methodfor generating a motion-corrected PET image of an examination area in acombined MR-PET system and a MR-PET system herefor.

BACKGROUND

The measurement of PET images of an examination area typically requiresmeasuring times in the range of minutes. As a result, moving body partscan only be measured out of focus. One possibility of improving thefocus in the generated PET images resides in using the so-called gating.With cyclical movements such as for instance respiratory movement orheart movement, the overall measuring time available is divided intoindividual phases of the movement. In order to create the PET image,only the PET events which occur in a specific phase of the movement aretaken into account. A sharp image, i.e. an image without motionartifacts, is herewith achieved. The price for this is however a reducedsignal-to-noise ratio, since the measuring time per image is reduced. Itis not necessary in many application instances to temporally resolve themovement of an organ in the examination area, but it is instead moreimportant for instance to be able to identify a lesion in the organ. Animage with an optimized signal-to-noise ratio is nevertheless requiredherefor. It can however only be achieved by lengthening the measuringtime.

SUMMARY

At least one embodiment of the present invention is directed toimproving the PET image measured in a specific measuring time such thatthe signal-to-noise ratio is improved.

Further embodiments are described in the dependent claims.

According to a first aspect of an embodiment of the invention, a methodis provided for generating a motion-corrected PET image of anexamination area in a combined MR-PET system. In a first step of themethod, PET events from the examination area are recorded in a firstrecording time frame. Furthermore, a number of MR images of theexamination area are recorded in the at least first recording timeframe. An at least two-dimensional item of movement information of theexamination area is then calculated on the basis of the number of MRimages. The movement information here describes the movement of theexamination area during the first recording time frame. Themotion-corrected PET image is then determined from the PET signals andby taking account of the calculated movement information. It is possiblein accordance with the invention to determine the movement of theexamination area in the first recording time frame with the highresolution of the MR images compared with PET images.

An embodiment of the invention further relates to a combined MR-PETsystem, which can generate the motion-corrected PET image. Thiscomprises inter alia a PET unit for recording the PET events from theexamination area in the first recording time frame and an MR unit forrecording the number of MR images. A computing unit is furthermoreprovided, which calculates at least two-dimensional movement informationof the examination area on the basis of the number of MR images. Thecomputing unit can also determine the motion-corrected PET image fromthe PET events using the calculated motion information.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are explained in more detail below withreference to the appending drawings, in which;

FIG. 1 shows a schematic representation of a combined MR-PET systemaccording to an embodiment of the invention,

FIG. 2 shows a schematic representation of the cyclical movement of theexamination area and the recording of the PET or MR data,

FIG. 3 shows the subdivision of the cyclical movement into individualtime segments, wherein PET images of the individual time segments arecorrected with the respective movement in the time segments,

FIG. 4 shows another possibility for determining the motion-correctedPET image, in which the individual PET events are directly taken intoaccount, and

FIG. 5 shows a flow diagram with yet another embodiment for calculatingthe motion-corrected PET image.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

The present invention will be further described in detail in conjunctionwith the accompanying drawings and embodiments. It should be understoodthat the particular embodiments described herein are only used toillustrate the present invention but not to limit the present invention.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the present invention to the particularforms disclosed. On the contrary, example embodiments are to cover allmodifications, equivalents, and alternatives falling within the scope ofthe invention. Like numbers refer to like elements throughout thedescription of the figures.

Specific structural and functional details disclosed herein are merelyrepresentative for purposes of describing example embodiments of thepresent invention. This invention may, however, be embodied in manyalternate forms and should not be construed as limited to only theembodiments set forth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments of thepresent invention. As used herein, the term “and/or,” includes any andall combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being“connected,” or “coupled,” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected,” or “directly coupled,” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between,” versus “directly between,” “adjacent,” versus“directly adjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments of the invention. As used herein, the singular forms “a,”“an,” and “the,” are intended to include the plural forms as well,unless the context clearly indicates otherwise. As used herein, theterms “and/or” and “at least one of” include any and all combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “comprises,” “comprising,” “includes,” and/or“including,” when used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, e.g., those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Spatially relative terms, such as “beneath”, “below”, “lower”, “above”,“upper”, and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, term such as “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein are interpreted accordingly.

Although the terms first, second, etc. may be used herein to describevarious elements, components, regions, layers and/or sections, it shouldbe understood that these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are used onlyto distinguish one element, component, region, layer, or section fromanother region, layer, or section. Thus, a first element, component,region, layer, or section discussed below could be termed a secondelement, component, region, layer, or section without departing from theteachings of the present invention.

According to a first aspect of an embodiment of the invention, a methodis provided for generating a motion-corrected PET image of anexamination area in a combined MR-PET system. In a first step of themethod, PET events from the examination area are recorded in a firstrecording time frame. Furthermore, a number of MR images of theexamination area are recorded in the at least first recording timeframe. An at least two-dimensional item of movement information of theexamination area is then calculated on the basis of the number of MRimages. The movement information here describes the movement of theexamination area during the first recording time frame. Themotion-corrected PET image is then determined from the PET signals andby taking account of the calculated movement information. It is possiblein accordance with the invention to determine the movement of theexamination area in the first recording time frame with the highresolution of the MR images compared with PET images.

When calculating the motion-corrected PET image from the PET events, themovement can then be determined and taken into account such that themovement of the examination area is corrected and a motion-corrected PETimage is calculated. It is herewith possible to take the PET events intoaccount even during the various positions during the movement of theexamination area. All PET events counted during a recording time framecan essentially be used for generating the motion-corrected PET image,and not only the PET events which were detected during a specificmovement state. The MR images are used to correct the movement arisingso that the PET events which took place during the movement can also betaken into account.

The movement of the examination area is preferably a cyclical movementsuch as for instance the respiratory movement or the heart movement. Inthis embodiment, the number of MR images is preferably recorded invarious time segments of the cyclical movement, as a result of which anitem of movement information is retained in the individual timesegments. The movement relative to a reference position of theexamination area is herewith preferably determined in individual timesegments. With the respiratory movement, this may be the position of theexamination area for instance (for instance in the abdomen) which hasthe examination area at the end of the exhalation phase. It is alsopossible to use any other reproducible position of the cyclicalmovement.

It is possible for instance to determine an at least two-dimensionalmovement segment information of the examination area for each of thetime segments, wherein each movement segment information describes themovement of the examination area in the associated time segment. Themovement which the examination area has made relative to a referenceposition can then be determined from the individual MR images in varioustime segments. Generally the at least two-dimensional movementinformation can contain a fixed translation of the examination area, arotation and/or a deformation of the examination area. Depending on themovement that has occurred, a rigid translation, a rotation and/or adeformation can be taken into consideration in the determination of themovement. The movement information can describe the movement in a planetwo-dimensionally, or it can be three-dimensional.

In one embodiment it is possible to generate time segment PET images forthe different time segments of the cyclical movement, wherein a timesegment PET image is generated on the basis of the PET events occurringin the associated time segment. The time segment PET image produced canthen be corrected with the associated movement segment information ofthe associated time segment so that a corrected time segment PET imageis produced for each time segment. The movement occurring in theassociated time segment is corrected herein. By totaling theindividually corrected time segment PET images, it is possible togenerate the motion-corrected PET image.

The individual time segment PET images can be weighted equally whenforming the total of the corrected time segment PET images for themotion-corrected PET image. In another embodiment, it is possible toimplement the total formation as a function of the size of the movementoccurring in a sub segment. This means that a time segment PET image isweighted less if the movement in the associated time segment was greaterthan in another time segment PET image, in which the movement was lowerin the associated time segment. This enables potential artifacts whichoccur if large transformations are needed to take the movement intoaccount in order to generate the time segment PET image to be reduced.

In another embodiment, the time segment PET images are not necessarilygenerated, but it is instead possible to correct the individual PETevents and the local information determined from the PET events suchthat the movement calculated from the MR images is taken into account.One possibility here is to determine the trajectory of the annihilationradiation associated with the respective PET event, that is followed bysaid annihilation radiation. With the aid of the at leasttwo-dimensional movement information, corrected trajectories which takethe movement of the examination area into account can then becalculated. The motion-corrected PET image can be calculated from thesecorrected trajectories. In this context it is possible to determinedeformed trajectories for the individual events on the basis of the atleast two-dimensional movement information which take the respectivemovement information into account and the movement-corrected PET imageis calculated with the aid of the deformed trajectories. The measuredPET events can herewith be distributed in accordance with the movementinformation for instance on new so-called lines of response. Afractional rebinning can herewith be used, which distributes a PET eventin accordance with the sum of the movement onto a deformed line ofresponse.

In order to determine the movement in the individual time segments, itis possible to record the MR images with a segmented recordingtechnology, in which the cyclical movement of the examination area ismonitored and a raw data space and/or k-space of an MR image belongingto a time segment is then only filled with raw data if a specificmovement state of the cyclical movement takes place.

It is furthermore possible to use the number of MR images, which arerecorded to determine the at least two-dimensional movement informationsimultaneously to determine the attenuation correction. The gammaradiation occurring in the PET detectors of the PET unit is attenuatedagain when passing through the examination area. With the aid of the MRimages, this attenuation can be better determined, since, with the aidof the MR images, it can be determined whether and which tissue liesbetween the source of the radiation and the PET detector. In thisembodiment, the recorded MR images can be used for different purposes,firstly for determining the attenuation correction and secondly for thecorrection of the movement.

An embodiment of the invention further relates to a combined MR-PETsystem, which can generate the motion-corrected PET image. Thiscomprises inter alia a PET unit for recording the PET events from theexamination area in the first recording time frame and an MR unit forrecording the number of MR images. A computing unit is furthermoreprovided, which calculates at least two-dimensional movement informationof the examination area on the basis of the number of MR images. Thecomputing unit can also determine the motion-corrected PET image fromthe PET events using the calculated motion information.

FIG. 1 shows a schematic embodiment of a combined MR-PET system 1. Thishas a MR-PET unit 2, with a magnet 3 for generating the polarizationfield B0 and the PET detectors 4. A person to be examined 14 arranged ona couch 5 can be introduced into the combined MR-PET unit 2. The personto be examined, of whom a PET image of the examination area 15 is to begenerated, can be injected with radionuclide by way of an injector 6,for instance 18F radioactively marked glucose. A movement detector 7 candetect the movement arising during the examination such as therespiratory movement or heart movement for instance. The movementdetector may be an EKG for instance with the heart movement, with therespiratory movement, the movement detector 7 can detect a marking onthe person to be examined, or the movement is detected with the aid ofthe generated MR images, for instance by determining the position of thediaphragm in the MR images. The system further comprises a PET unit 8,which is able to calculate a PET image with the aid of the eventsdetected in the PET detectors 4. As a PET image can be generated fromthe PET detectors 4 with the aid of the signal, it is essentiallyfamiliar to the person skilled in the art and is not described in moredetail here.

An MR unit 9 is likewise provided, which produces MR images from the MRsignals by a detection coil (not shown). How MR signals can be generatedand detected by high frequency pulses for deflecting the magnetizationand by switching magnetic field gradients is familiar to the personskilled in the art and is not explained in more detail here. It islikewise known to the person skilled in the art that the two units, theMR unit 9 and the PET unit 8, can be operated in a combined unit, asshown. A control unit 11 is provided to control the entire system. Thegenerated MR or PET images or other control information can be displayedon a display unit 13, wherein a user of the system can control theMR-PET system by way of an input unit 12.

A computing unit 10 calculates a motion-corrected PET image as explainedbelow.

A cyclical movement of the examination area is now shown in FIG. 2, ascan occur during the recording of the PET events. In the case shown, thecyclical movement 20 is a respiratory movement for instance as wasdetected by the movement detector 7. Furthermore, a first recording timeframe 25 is shown, during which the PET events are recorded. These PETevents during the recording time frame 25 can be stored as a function oftime. In this so-called list mode, the individual PET events are storedwith the associated time when the PET event took place. By temporaltotaling during the first recording time frame 25, PET images can thenbe generated. According to the prior art, it was possible for instanceonly to take PET events into account in the time frame 21, here theexhaled state. With a given recording time span, the signal-to-noiseratio is herewith nevertheless lower, since not all PET events can betaken into account or the recording time frame has to be lengthenedaccordingly in order to be able to take a corresponding number of PETevents into account.

As explained below, it is now possible in accordance with the inventionto take all PET events into account during the first recording timeframe 25. A number of MR images are recorded throughout a second timeframe 26. In the example shown, the second recording time framecorresponds to the first recording time frame. The time frame 26 may belonger than the first time frame 25. MR images are however likewiserecorded in the first time frame 25.

The recorded MR images can be generated for instance with a segmentedrecording technology. With this segmented recording technology, thek-space associated with an MR image is only then ever filled with MRsignals if a specific movement segment of the cyclical movement takesplace. For instance, a segmented gradient echo sequence can be used forthe MR imaging.

With regard to FIG. 3, this means for instance that the movement segmentis divided into a number of time segments, in the case shown in FIG. 3,the respiratory movement was subdivided into six different sub segments.The segmented recording technology can now be used such that sixdifferent MR recordings 31-36 are recorded for instance, wherein each MRrecording only takes signals into account from a specific time segmentof the movement. In the case shown, only MR signals are detected andtaken into account for instance for creating the MR image 31 if theexamination area is located in the second time segment and the MR image36 only if the examination area is disposed in the time segment 6. It isthus possible to determine the movement of the examination area relativeto a reference position.

As a reference position, the position of the examination area and/or theorgan contained therein can be at the end of the exhalation cycle, i.e.for instance the time segment 1 in the exemplary embodiment of FIG. 3.For the individual MR images 32-36, the movement relative to thisreference position can be determined. The movement may be for instance atranslation, rotation and/or deformation. Movement segment information41-46 can be generated herefrom for each of the time segments. Themovement segment information contains in each instance the movement inthe associated time segment relative to the reference position. If thetime segment 1 is the reference position for instance, then the movementsegment information 41 is zero and the other movement segmentinformation 42-46 describes the movement of the examination arearelative to the position of the examination area, which it holds duringthe time span 1. Upon selection of the reference position, a position ispreferably used which can be clearly reproduced.

It is likewise possible to use the PET events associated with the timesegments in order to create a time segment PET image in the individualtime segments, here the time segments 1-6. For instance, only the PETevents were taken into account for the time segment PET image 51 whichoccur during the first recording time frame 25 and in each instance inthe first time segment of the cyclical movement. Accordingly, the timesegment PET images 52-56 are generated. The previously calculatedmovement segment information 41-46 can then be applied to the respectivetime segment PET images 51-56 in order in each instance to generate acorrected time segment PET image 61-66. The movement segment informationmay be a two or three-dimensional vector field for instance, whichdescribes the movement of the examination object relative to a referenceposition. In the corrected time segment PET image 61-66, the relativemovement of the associated time segment occurring relative to thereference position has then to be taken into account and corrected. Inthe afore-cited example with the reference position at the end of theexhalation cycle, this would mean that all images 61-66 indicate a PETimage of the examination area with a position of the examination area asin the first time segment. If the first sub segment is the referenceposition for instance, the PET images 62-66 indicate the examinationarea in the position of the first time segment, since the movementoccurring relative to the first time segment was corrected. It is thenpossible to total the individual time segment PET images in order toform the motion-corrected PET image 70.

The PET image can generally also be used as a basis for the total image,i.e. the reference position, which already comprises the beststatistics, i.e. the PET image, which has the majority of PET eventswithin the different time segments of the cyclical movement. When thetotal required to generate the motion-corrected PET image 70 is formed,the individual time segment PET images 61-66 can be easily totaled. Itis furthermore also possible to weight the total as a function of thesize of the movement correction. In other words, the greater a movementcorrection was for generating the time segment PET images, the lower itsweighting can be in the total image, in order to reduce the inaccuraciesoccurring as a result of the movement correction.

A further embodiment is shown in FIG. 4. It is not necessary in thisembodiment to generate the time segment PET images associated with therespective time segments. Instead, the PET events and the localinformation calculated during the PET event are corrected directly. Onceagain the cyclical respiratory movement is shown in the upper part inFIG. 4, wherein, as in FIG. 3, movement segment information 41-46likewise exists, which describes the movement of the examination area inthe associated time segment relative to a reference position. The PETevents 81-84, which appear distributed randomly during the recordingtime span 25, are detected. The β+-decay occurring during a PET eventresults, as known, in an annihilation of the positron and the electron,wherein the annihilation beams arising, the two γ-quanta, are irradiatedin diametrically opposite directions. The original position of theβ+-decay lies on the so-called line of response. Two such lines 91 and92 are shown in FIG. 4. The line 91 is shown in FIG. 4 for the PETevent, wherein it was calculated for instance that the β+-decay has totake place at the position x1. In the example shown, the time segment 1is in turn the reference position, so that the movement relative to thereference position is determined. The β+-decay 83 arising in timesegment 4 is however detected if the examination area has moved. It isestimated from the associated line 92 for instance that the decay mustin turn have taken place at the position x1. It is however known fromthe movement segment information which was determined on the basis ofthe MR images that the tissue, which was found in time segment 4 atposition x1, was actually found in time segment 1 at x2. If thedifferent PET events are now to be totaled directly, this movement canbe taken into account such that on account of the MR images, it is knownthat the tissue which was responsible for the β+-decay and which ismirrored in the PET event 83, is the same tissue which is in thereference time segment at position x2, as shown symbolically by thearrow. This means that with the exemplary embodiment shown in FIG. 4,the measured PET events distribute corresponding movement information onnew lines (line of response). To this end, a fractional rebinning isinter alia necessary, which distributes an integral PET event accordingto the sum on a deformed line, in FIG. 4 in the example shown on line93.

It is basically possible to reconstruct the 3D PET data recording with amulti ring PET detector by means of a 3D projection. Alternatively tothis, the 3D data records can be subjected to a so-called rebinningalgorithm. The basic principle here is to calculate and redistribute the3D data record in an equivalent 2D data record. This can bereconstructed with a correspondingly lower time outlay using anestablished 2D reconstruction method, such as for instance the filteredreproduction.

FIG. 5 shows a flow diagram, which shows the steps for calculating amotion-corrected PET image according to an embodiment of the invention.In a first step, the PET and MR data are detected for instance (stepS1). It is further possible to use the existing MR images not only formovement correction but instead also for calculating an attenuationcorrection. The photons arising at a specific point in the tissue passthrough tissue and air prior to exiting onto the detector, whereindifferent tissue types can be passed through. With the aid of thecalculated MR images, it is possible in a good resolution to determinethe tissue-specific attenuation correction on the path of the photon tothe detector. This means that an attenuation correction can becalculated in step S2 with the aid of the MR data. The same MR images,which are used in step S2, can now be used to calculate a movementcorrection, as was described in detail in FIG. 3 or 4. It is thenpossible to calculate a motion-corrected PET image with a goodsignal-to-noise ratio in step S4.

As can be concluded from the above said, the PET events are detectedthroughout the first time frame. Further, the MR images are recorded atleast throughout this first time frame 25. When the detection of the PETevents and the acquisition of the MR images is accomplished, it ispossible to analyze the cyclic movement, e.g. the respiratory motion.The cyclic movement can then be divided into different time segments sothat in each time segment a certain movement has occurred. It is thenpossible to assign the different PET events which have occurred duringthe first time frame to the different time segments and to generate arespective time segment PET image using the PET events occurring in theassociated time segment. With the present invention, a retrospectivecorrection of the movement is possible as the cyclic movement isdetermined over the first time frame and based on the patient'srespiratory motion. The segmentation into the different time segmentscan take into account the motion pattern for the examined patient. Asthe cyclic movement can vary from patient to patient, the cyclicmovement can be determined for the examined patient and the motioncorrection is carried out taking into account the specific motion thatoccurs during the first time frame where the PET events are recorded.This improves the overall correction of the motion in the PET image.

In summary, a embodiment of the invention allows a PET image to begenerated with a high signal-to-noise ratio, since essentially all PETevents can be used to generate the PET image and the movement producedduring the recording time frame is taken into account. The generated MRimages are preferably likewise used to determine the attenuationcorrection.

One possibility of how a translation, rotation or deformation of atissue can be determined from the measured MR images, is described forinstance in Torsten Rohlfing et al. in “Modeling liver motion anddeformation during the respiratory cycle using intensity-based free-formregistration of gated MR images”, proceedings of SPIE, volume 4319,2001, the entire contents of which are hereby incorporated herein byreference.

What is claimed is:
 1. A method for generating a motion-corrected PETimage of an examination area in a combined MR-PET system, the methodcomprising: recording PET events from the examination area in a firstrecording time frame; recording a number of MR images of the examinationarea in at least the first recording time frame; calculating an at leasttwo-dimensional movement information of the examination area on thebasis of the recorded number of MR images, wherein the at leasttwo-dimensional movement information describes the movement of theexamination area during the first recording time frame, the movement ofthe examination area being a cyclical movement and the number of MRimages are recorded in different time segments of the cyclical movement;and determining the motion-corrected PET image from the recorded PETevents using the calculated at least two-dimensional movementinformation, the determining including, generating time segment PETimages for the different times segments of the cyclical movement, arespective time segment PET image of the time segment PET images isgenerated on the basis of the PET events occurring in the associatedtime segment, the generated time segment PET images being associatedwith movement segment information of the at least two-dimensionalmovement information, and correcting the time segment PET images withthe associated movement segment information such that a corrected timesegment PET image appears for each time segment of the cyclicalmovement, in which the movement occurring in the associated time segmentis corrected.
 2. The method of claim 1, wherein at least two-dimensionalmovement segment information of the examination area is determined foreach of the time segments, which describes the movement of theexamination area in the associated time segment.
 3. The method of claim1, wherein the determining includes, summing the corrected time segmentPET images to generate the motion-corrected PET image.
 4. The method ofclaim 3, wherein the summing is based on sizes of the movement occurringin the time segments, and is the time segment PET images are weightedrelatively lower in the summation if the movement was relatively largerin the associated time segment than another time segment PET image, inwhich the movement was relatively lower in the associated time segment.5. The method of claim 1, further comprising: determining an associatedtrajectory followed by an annihilation radiation is for each PET event,and the determining the motion-corrected PET image includes, calculatingcorrected trajectories for the PET events with the aid of thetwo-dimensional movement information, and determining themotion-corrected PET image from the corrected trajectories.
 6. Themethod of claim 5, wherein the determining the associated trajectorydetermines deformed trajectories for the PET events on the basis of theat least two-dimensional movement information of the examination area,and the determining the motion-corrected PET image includes, determiningthe motion-corrected PET image on the basis of the deformedtrajectories.
 7. The method of claim 1, wherein the at least twodimensional movement information describes at least one of atranslation, rotation and deformation of the examination area.
 8. Themethod of claim 1, wherein the number of MR images is further used todetermine an attenuation correction, which is used when determining themotion-corrected PET image.
 9. A method for generating amotion-corrected PET image of an examination area in a combined MR-PETsystem, the method comprising: recording PET events from the examinationarea in a first recording time frame; recording a number of MR images ofthe examination area in at least the first recording time frame;calculating an at least two-dimensional movement information of theexamination area on the basis of the recorded number of MR images,wherein the at least two-dimensional movement information describes themovement of the examination area during the first recording time frame;and determining the motion-corrected PET image from the recorded PETevents using the calculated at least two-dimensional movementinformation, wherein the number of MR images is recorded with asegmented recording technology, in which a cyclical movement of theexamination area is monitored and a raw data space of an MR imagebelonging to a time segment is then only filled with raw data if aspecific movement state of the cyclical movement takes place.
 10. Acombined MR-PET system, configured to generate a motion-corrected PETimage of an examination area, the combined MR-PET system comprising: aPET unit configured to record PET events from the examination area in afirst recording time frame; an MR unit configured to record a number ofMR images of the examination area in at least the first recording timeframe; and a computing unit configured to calculate an at leasttwo-dimensional item of movement information of the examination area onthe basis of the number of MR images, wherein the movement informationdescribes the movement of the examination area during the firstrecording time frame, the movement of the examination area being acyclical movement and the number of MR images are recorded in differenttime segments of the cyclical movement, and wherein the computing unitis further configured to determine the motion-corrected PET image fromthe PET events using the calculated movement information, the computingunit further configured to, generate time segment PET images for thedifferent times segments of the cyclical movement, a respective timesegment PET image of the time segment PET images is generated on thebasis of the PET events occurring in the associated time segment, thegenerated time segment PET images being associated with movement segmentinformation of the at least two-dimensional movement information, andcorrect the time segment PET images with the associated movement segmentinformation such that a corrected time segment PET image appears foreach time segment of the cyclical movement, in which the movementoccurring in the associated time segment is corrected, themotion-corrected PET image being based on the corrected time segment PETimages.